In this review, we discuss the ecological and evolutionary
consequences of plant-herbivore interactions in tropical forests. We
note first that herbivory rates are higher in tropical forests than in
temperate ones and that, in contrast to leaves in temperate forests,
most of the damage to tropical leaves occurs when they are young and
expanding. Leaves in dry tropical forests also suffer higher rates of
damage than in wet forests, and damage is greater in the understory
than in the canopy. Insect herbivores, which typically have a narrow
host range in the tropics, cause most of the damage to leaves and have
selected for a wide variety of chemical, developmental, and
phenological defenses in plants. Pathogens are less studied but cause
considerable damage and, along with insect herbivores, may contribute
to the maintenance of tree diversity. Foliovorous mammals do less
damage than insects or pathogens but have evolved to cope with the
high levels of plant defenses. Leaves in tropical forests are defended
by having low nutritional quality, greater toughness, and a wide
variety of secondary metabolites, many of which are more common in
tropical than temperate forests. Tannins, toughness, and low
nutritional quality lengthen insect developmental times, making them
more vulnerable to predators and parasitoids. The widespread
occurrence of these defenses suggests that natural enemies are key
participants in plant defenses and may have influenced the evolution
of these traits. To escape damage, leaves may expand rapidly, be
flushed synchronously, or be produced during the dry season when
herbivores are rare. One strategy virtually limited to tropical
forests is for plants to flush leaves but delay "greening" them until
the leaves are mature. Many of these defensive traits are correlated
within species, due to physiological constraints and tradeoffs. In
general, shade-tolerant species invest more in defenses than do
gap-requiring ones, and species with long-lived leaves are better
defended than those with short-lived leaves.